Because of its non-invasive/non-destructive nature, magnetic resonance techniques represent powerful approaches to a host of basic and clinical biomedical research problems. Nuclear magnetic resonance (NMR) in particular has seen an explosion in applications to in vivo systems ranging from small animal models to man. Nevertheless, NMR applications are severely restricted by extremely poor signal-to-noise sensitivity. For many applications, electron paramagnetic resonance (EPR) offers greatly improved SIN if major technical problems associated with large conductive samples can be solved. For example, in vivo EPR experiments at standard microwave frequencies suffer extreme microwave sample-- penetration limitations and antenna sensitivity damping. Herein we propose an unorthodox EPR spectrometer design utilizing fast field switching (FFS) that if successful will enable routine low field (70 Gauss/200 MHz) EPR experiments in vivo. Operation at radio frequencies greatly reduces conductive sample related technical difficulties. This design, by avoiding microwave interrogation pulses, allows for extremely fast receiver recovery with spectrometer dead times less than 10 ns expected. This will allow a wide range of electron spin labels to be used. The short spin lattice relaxation times of electron spins will allow very efficient time averaging further increasing S/N. The FFS-EPR design makes use of only recently available high current fast switching transistors. We know of no other laboratory currently pursuing this design strategy. If successful this approach to in vivo EPR will make accessible to magnetic resonance techniques an entirely new regime of biomedical research problems.